With presently known LTE (Long Term Evolution) cellular telecommunications networks—especially when channel centre frequencies are aligned and/or frequency channels have overlapping bandwidth—typically there exist interference problems, especially in edge regions of cellular telecommunications networks, i.e. in the region of overlap of two cellular telecommunications networks that are adjacent to each other. This typically occurs at the edge region of the radio coverage area of cellular telecommunications networks in the vicinity of a territorial border, e.g. between different countries.
Therefore, radio frequency cells and sites of such cellular telecommunications networks of a mobile network operator have to pass several approval-processes, typically of the national regulatory authority, such as, e.g. in Germany, Bundesnetzagentur BNetzA. One of such requirements that the cellular telecommunications network has to fulfill is that the radio transmissions of a radio frequency mobile network cell need to be checked in order to assure that such transmissions do not cover areas of the adjacent geographical area (typically a foreign country) such that predefined field-strength thresholds are exceeded.
Typically, this check is based, e.g., on the HCM-agreement, defined by 17 neighboured European countries. In the context of the HCM-agreement, a common prediction model is used—the so-called Harmonised Calculation Method (HCM)—which is used to predict the field-strength of a radio cell at different distances from the border line, such as, e.g., 0 km (from the border line, i.e. on the border) or 6 km (from the border line in the respective adjacent country). If the predicted thresholds are not violated or exceeded, it is assumed that no interference is generated within the respective other cellular telecommunications network (i.e. the cellular telecommunications network of the foreign country) and the considered radio cell is allowed to be permanently emitting (or being active or “on air”).
However, as the HCM-agreement aims at avoiding situations of interference, the application of the HCM-agreement and the respective field strengths has the consequence that often the edge region of the radio coverage area of the cellular telecommunications network, i.e. typically the area of the border between two countries, is not sufficient covered with respect to LTE service.
Moreover, the prediction model according to the HCM-agreement is a mathematical model and does not perfectly fit real situations in practice. Wherever real interference situation appears, especially in cases where no mobility between the mobile operators is agreed and configured (e.g. as a part of a local limited network sharing according to document EP 2 213 128 B1), performance degradations will probably appear.
These degradations are extremely strong in case that the same or similar downlink centre frequency and physical cell identifier (PCI) are used in cells that belong to different cellular telecommunications networks but are adjacent along respective edge regions of the cellular telecommunications networks (typically along the territorial border between countries). As a solution to this problem, e.g., the ECC (Electronic Communications Committee within the European Conference of Postal and Telecommunications Administrations) recommends (cf. document “ECC Recommendation04”) a coordination of physical cell identifiers between the different cellular telecommunications networks along a territorial border if they operate their mobile telecommunication networks with the same or similar downlink centre frequencies.
Additionally, the 3GPP standardization offers options to blacklist physical cell identifiers for the purpose of measurements needed for idle mode procedures (namely cell-re-/selection as defined for LTE intra-frequency and LTE inter-frequency in 3GPP TS 136.331) and active mode procedures, e.g. for IntraLTE mobility, in a serving cell. Within the context of an overlapping mobile network with a foreign network operator, with the same used LTE centre frequencies, with the blacklisting of physical cell identifiers it can be achieved to avoid unnecessary user equipment measurements for the purpose of cell-reselection (idle mode procedure) and/or event and/or periodic reports of cells (i.e. physical cell identifiers with their received field strengths and/or measured quality) of the foreign network operator, i.e. of the cellular telecommunications network on the other side of the territorial border considered.
Typically, interference relations between a pair of cells are not symmetrical in downlink, i.e. a first radio cell can strongly interfere with a second radio cell but this does not necessarily mean that the second radio cell strongly interferes with the first radio cell). The consequence is that currently, there is no set of rules available how exactly define physical cell identifier blacklist entries in the different radio cells (i.e. for the definition of where to set the physical cell identifier blacklist entries into the serving cells of the own cellular telecommunications network. Often, an exchange of network configuration data between mobile operators operating adjacent cellular telecommunications networks—e.g. to calculate, based on a prediction, the interference relations between adjacent cells of different cellular telecommunications networks—is not possible or not desired.
Beside the HCM-agreement, there might be additional agreements of national authorities, e.g. allowing increased thresholds of field strengths in order to increase the coverage probability along the territorial border. Typically, these bilateral agreements of the national authorities further authorize the mobile network operators to agreed additional operator-specific agreements.
However, currently a sufficient LTE-coverage near the territorial border or in the edge region of the radio coverage area of the cellular telecommunications network is quite hard to achieve and is in several cases limited through theoretical frequency co-ordination models, as HCM in Germany and other European countries. There are at least two kind of weaknesses, namely an under-prediction or an over-prediction at the different distances from the border (i.e. x km distances of, e.g. 0 km or 6 km from the borderline), which leads to either too less LTE coverage in reality or to a real interference situation between adjacent radio cells of the different cellular telecommunications networks even that this was not predicted by the used prediction model.
Especially the second case involves a lot of effort, as the detection of the reason of this interference has to be typically found out by expensive drive tests, e.g. a special car with a radiofrequency scanner and signalling decoder. The first case (lack of real LTE-coverage) can often not be finally solved, because of the used underlying theoretical prediction model within the official approval process of the respective national regulatory authority.